The present invention relates to apparatus and methods for drying a continuous tissue paper web in a papermaking machine. The invention relates more particularly to a drying section and method in which the web is passed through a pre-drying section and a final drying section, wherein the pre-drying section has at least one press that includes a first press device and a counter roll arranged in contact with each other so as to form a press nip there between, and an endless fabric is arranged to pass through the press nip together with the web.
The term xe2x80x9ctissuexe2x80x9d as used herein is intended to include all kinds of soft hygiene paper, whether creped or not, including but not limited to handkerchief paper, bathroom tissue, and towel.
In the production of soft paper or tissue at lower basis weights, which is used for the production of household paper such as paper towels and other sanitary products, it is a general requirement that the bulk, i.e., the ratio between the volume and the weight of the paper, should be substantially higher than for other types of paper. Paper with a high bulk exhibits a desired combination of softness and high water absorption capacity.
The present commercially used technology to dry a continuous tissue web in a drying section employs large-diameter cylinders, so-called Yankee dryers, which are steam heated internally, or employs porous cylinders (so-called TAD cylinders) on which thermal through-air drying of the web is performed by blowing hot air through the web, either outwardly from the inside of the cylinder or inwardly from the outside of the cylinder.
Use of Yankee dryers for drying tissue has a major disadvantage in that the smooth surface of the cylinder causes the web to be compressed as it is pressed onto the Yankee dryer by means of a press roll, which is wrapped by a felt that carries the web to the Yankee dryer. This is especially the case since the web generally is still relatively wet when it is transferred to the Yankee dryer, leading to its structure and bulkiness being impaired by the treatment on the Yankee dryer. Hence, in a conventional drying section for tissue, only limited bulk levels can be reached. Typically, the web is creped from the Yankee dryer in order to somewhat improve its softness and bulkiness, but generally that improvement is lost when the tissue is used and thereby becomes wet.
In the development of drying sections for tissue, it has also been suggested to pre-dry the web in a press section comprising one or more press nips, possibly of shoe press type. The pre-dried web is then finally dried on a conventional drying cylinder such as a Yankee dryer. A papermaking machine of this type, including a pre-drying section and a final drying section, is disclosed in U.S. Pat. No. 5,393,384, the web being carried by an impermeable belt through the drying sections. Although a certain beneficial effect results from this type of system, the pre-drying section cannot be made adequately efficient, since only limited linear loads can be used in the press nips in order to avoid compressing the tissue web and thereby reducing the bulk of the web. Consequently, one press is usually not sufficient to reach the desired dry content levels in the pre-drying section, and hence at least two presses have to be used, which requires more space, investment costs and energy. Despite using at least two presses, the resulting tissue becomes relatively flat and compact, with a bulk that is lower than the desired high bulk. Another problem with the use of such presses is that rewetting of the web at the outlet from the press nip usually occurs, which reduces the effectiveness of the press in dewatering the web.
Drastically improved bulk levels have been reached by the use of thermal through-air drying cylinders, i.e., TAD cylinders. Typically, the bulk is increased by about 60-200%, compared to the bulk levels that are reached with a conventional press nip followed by a Yankee dryer. Usually, the TAD cylinder is preceded by a suction apparatus, by which an imprinting pattern is achieved in the web in its wet state. The web is thereafter dried in the TAD cylinder, by inward or outward air flow, while retaining the structure of the imprinting pattern and thereby retaining the high bulk of the web. In the TAD drying, the imprinted web structure is, in a matter of speaking, frozen in its structure.
The TAD cylinder, however, has the disadvantages that it requires a lot of space, has a limited capacity, and yet requires a lot of energy. Moreover, the TAD cylinder requires the use of very large air volumes that have to be handled. Also it entails high investment costs.
Another known method to dry a fibrous web is to use so-called impulse drying. The wet web is pressed at a high temperature and with a high linear load in a press section comprising one or more press nips. The technique is described in SE 7803672-0, corresponding to U.S. Pat. No. 4,324,613, and is used in web types other than tissue webs, or for tissue webs with limited bulk. In these patents, there are described maximum specific pressures of 3-8 MPa and surface temperatures on the counter roll of a press of conventional type of between about 150xc2x0 C. and 350xc2x0 C. By xe2x80x9cconventional typexe2x80x9d is meant a press nip in which two rolls with cylindrical cross sections are counter-acting against each other under pressure. The time that a given region of the moving web resides in this type of press nip, however, is only a few milliseconds because of the short length of the press nip in the machine direction, which is too short a time for the beneficial effects of the high pressure acting at a high temperature to be fully developed. Therefore, it has also been suggested to use impulse drying in a heated shoe press in which the press nip is extended to about 20-30 cm, giving much longer residence times for the web in the nip.
U.S. Pat. No. 5,556,551 discloses such an impulse drying process for drying paper webs such as toilet paper. The web and a water absorbing felt are fed into a press nip of a shoe press. The web may be heated by steam prior to its entry into the nip, or the smooth surface of the counter roll may be heated prior to the nip. Delamination, caused by the sudden expansion of flash steam when the web leaves the high-pressure nip, is said to improve the volume and softness of the web. The dried web is creped from the counter roll by a scraper. High bulk levels cannot be reached by the process of U.S. Pat. No. 5,556,551, since the web will be compressed in the nip between the absorbing felt which has a flat surface and the smooth surface of the counter roll.
Yet another known method to dry tissue web is to use a drying cylinder, such as a Yankee dryer, as the counter roll in a press nip. The roll that co-acts with the counter roll may also be equipped with a press shoe so that an extended nip is formed. Systems of this type are shown in DE 196 54 345 (FIG. 5) and DE 43 21403 (FIG. 11). The press shoe may include a heating device. In U.S. Pat. No. 3,806,406, there is shown a system for the formation of a tissue web, which system also is of the type with a press nip between a first roll and a Yankee dryer. This system seeks to avoid compressing of the high-bulk tissue web by providing the surface of the Yankee dryer with a relief pattern with depressions and elevated parts therebetween. Essentially, only the parts of the web that abut the elevated parts of the cylinder surface are pressed together in the press nip, the intermediate parts being relatively unaffected. The web is thereby provided with an imprinting pattern consisting of parts that are pressed together and other parts that are not pressed together, corresponding to the pattern of the cylinder surface.
Despite several methods, devices and systems being known for drying tissue webs, there is no commercially available drying section that gives a high bulk, high water absorption levels, a good softness, and a distinct imprinting pattern in the web, while also requiring only a relatively small space, entailing relatively low investment and energy costs, and yet having a high capacity and good reliability.
The above needs are met and other advantages are achieved by the present invention, which provides a drying section for drying a web in a papermaking machine, the drying section comprising a pre-drying section and a final drying section, wherein the pre-drying section includes a hot press formed by a first press member and a rotatable counter roll in engagement with each other so as to form a nip therebetween through which the web passes. The hot press further includes a heating device disposed in heat-transfer relation to the counter roll and operable to heat a region of the exterior cylindrical surface of the counter roll, which then passes through the nip so as to heat the web therein, and an imprinting fabric arranged in an endless loop, the imprinting fabric defining an imprinting surface for imprinting the web and being arranged to pass through the nip of the hot press with the web against the imprinting surface such that the web is imprinted. The imprinting fabric continues to support the imprinted web downstream of the hot press at least up to the final drying section.
Since the terms xe2x80x9cimpulse dryerxe2x80x9d and xe2x80x9cimpulse dryingxe2x80x9d imply that water in the web evaporates so violently that the formed steam blows out liquid water from the web, the more generic terms xe2x80x9chot pressxe2x80x9d and xe2x80x9chot pressingxe2x80x9d are used to describe the present invention.
Preferably, the surface region of the counter roll is arranged to be heated by an external heating element to a temperature of at least 150xc2x0 C., and more preferably 200-300xc2x0 C. The external heating element is arranged at a location other than where the web is carried by the counter roll, preferably just ahead of the press nip.
In one embodiment, the imprinting fabric is arranged to maintain the web against the counter roll over a sector of the counter roll subtending an angle of about 90xc2x0 to 300xc2x0. Accordingly, the web remains in contact with the heated surface of the counter roll for an extended period of time so that the drying of the web is performed for a greater time. The hot press can also include a ventilation hood surrounding a portion of the sector of the counter roll wrapped by the web for carrying away steam liberated from the web. Alternatively, the pre-drying section can be arranged to lead the web away from the first press device and the counter roll essentially immediately downstream of the press nip. One advantage of this arrangement is that a higher temperature may be used on the counter roll without risk of the (synthetic) fabric being destroyed.
Preferably, in every location in the drying section where the web is pressed or subjected to a load between two surfaces, such as between the surface of a roll or cylinder and a fabric or clothing, or between two surfaces in a press nip, at least one of said two surfaces exhibits a structured/coarse surface. In the case of the structured/coarse surface being a fabric or a clothing, it preferably is a permeable imprinting fabric or clothing. In the case of the structured/coarse surface being the mantle surface of a roll or cylinder, it is a surface with a relief pattern. In this way, the web is never completely compressed between two flat or smooth surfaces in the drying section, whereby it retains a high bulk, a good softness and good water absorption levels, the structure of the web being essentially retained even when it is re-wetted when used.
The first press member can be a cylindrical roll or can include a press shoe forming an extended nip with the counter roll. Preferably, the drying section also includes a clothing operable to carry water away from the web. The clothing is arranged in an endless loop such that the clothing passes through the nip with the imprinting fabric sandwiched between the clothing and the web and then separates from the imprinting fabric and the web downstream of the nip.
The drying section can also include at least one suction device disposed upstream of the nip for suctioning the web against the imprinting surface of the imprinting fabric such that the web is made to conform to the imprinting surface.
The final drying section in some embodiments includes a heated dryer roll, and the imprinting fabric loop includes a guide roll that forms a nip with the heated dryer roll through which the imprinting fabric with the web supported thereon passes. The web is transferred from the imprinting fabric onto the heated dryer roll at the nip, and the web is then carried about the heated dryer roll for final drying of the web. Since the web is pressed onto the heated dryer roll by the imprinting fabric, the web is not compressed between two smooth surfaces, and hence the imprinted web is finally dried in its imprinted condition, which tends to lock the imprinted structure into the web so that it remains even upon re-wetting of the web.
In other embodiments, the final drying section includes a rotary through-air dryer having a drying cylinder about which the imprinting fabric is wrapped so as to carry the web about the drying cylinder. In yet other embodiments of the invention, the final drying section includes a rotary through-air dryer having a drying cylinder and further includes a drying fabric arranged in an endless loop so as to pass over the drying cylinder. The imprinting fabric loop is arranged to transfer the web from the imprinting fabric onto the drying fabric upstream of the drying cylinder, and the drying fabric carries the web about the drying cylinder.
The counter roll may be a solid cylindrical body, or may comprise a heatable, rotatable sleeve that surrounds a rotatable roll body. A region of the exterior cylindrical surface of the sleeve is heated by an external heating element. The sleeve preferably loosely surrounds the roll body in order to allow free expansion of the sleeve in relation to the roll body as the sleeve is heated. The sleeve and the roll body are eccentrically arranged in relation to each other such that the sleeve is in contact with the first press device in the press nip and also is in contact with the roll body in the press nip, so that a linear load is conveyed from the roll body to the nip via the sleeve. The rotation of the sleeve is driven by the rotation of the roll body, the friction between the roll body end the sleeve forcing the sleeve to rotate with the same peripheral speed as the roll body. By this construction, temperature-related stress in the counter roll is substantially avoided. Such stress may occur with a solid cylindrical counter roll because the outer layer of the roll becomes hotter than the inner layers of the roll, and hence the outer layer will expand more than the inner layer, resulting in temperature-induced stresses. Also, with a solid roll it may be difficult to maintain the desired geometrical shape of the roll across the machine direction because of difficulties in maintaining the same temperature in the mantle surface as in the gables. With a sleeve that is loosely arranged around the roll body, these types of problems are avoided. Another advantage with the exchangeable sleeve is that the imprinting pattern may be changed relatively simply, by changing sleeves.
Preferably, the hot press is arranged to provide an outgoing dry content of 40-60%, and more preferably 45-55%, in the web, when the incoming web to the press has a dry content of 10-30%, and more preferably 20-30%. Accordingly, the dry content of the web is raised 10-50% in the hot press nip. Although not preferred, it is possible to use more than one press nip in the pre-drying section, the additional press nip(s) being of conventional wet press type or of impulse drying type.
The press nip is preferably arranged to provide a linear load of 200-800 kN/m, and more preferably 300-600 kN/m, between the first press device and the counter roll. The counter roll may have a smooth surface, at least if the fabric that carries the web through the nip has a structured, imprinting surface. Alternatively, the mantle surface of the counter roll may have a patterned, imprinting surface, with depressions with a depth of preferably 0.1-2.0 mm, and parts between the depressions, i.e. elevated parts, preferably having a width of 1-5 mm. The surface area of the elevated parts, i.e. the surface area which will be in contact with the web, constitutes 20-50% of the total surface area of the mantle surface, measured at the circumference of the mantle. If both the permeable fabric and the counter roll are provided with imprinting patterns, the tissue product will beneficially be imprinted on both sides. Preferably, the counter roll has a diameter of 2.0-3.6 m.
According to another aspect of the invention, pressurized air knives or the like are arranged to release the web from the surface of the counter roll.
By the combination, according to the invention, of a pre-drying section based on hot pressing technique and a final drying section that may include a drying cylinder such as a Yankee dryer or a TAD cylinder or other drying means known in the art, and with the web never being compressed between two flat or smooth surfaces in the drying section, a tissue web is produced with very high bulk levels and with structural properties very similar to the structural properties of TAD-dried tissue webs. The tissue web is pre-dried to a relatively high dry solids content before the final drying of the web, which means that the imprinted structure of the web will not be impaired when it is conveyed to a drying cylinder such as a Yankee dryer. At the same time, the energy demand is decreased in comparison with the energy demand for TAD drying, and the investment cost and space requirement is lowered. The efficient drying in the pre-drying section, ensured by the press nip with hot pressing, makes it possible to increase the speed of the tissue paper machine. Alternatively, smaller-diameter drying cylinders can be used in the final drying section; in this case of a TAD cylinder used in the final drying section, this means that smaller air flows are required. When the present invention is applied to the reconstruction of an existing papermaking machine, the capacity of the machine may be increased without requiring any extra space, and also the efficiency and reliability of the machine can be improved.